Epoxy resins are a group of synthetic resins used to make plastics and adhesives. The viscosity, epoxy equivalent weight and molecular weight are the important properties of epoxy resins. These materials are of great importance because of their formulating and processing versatility. Epoxy resins can be used as coating agents because of their high resistance to chemicals and outstanding adhesion, durability, and toughness. Because of their high electrical resistance, durability at high and low temperatures, and the ease with which they can be poured or cast without forming bubbles, epoxy resins are especially useful for encapsulating electrical and electronic components. Epoxy resin adhesives can be used on metals, construction materials and synthetic resins. They are strong enough to be used in place of rivets and welds in certain industrial applications.
There are two main categories of epoxy resins, namely the glycidyl epoxy, and non-glycidyl epoxy resins. The glycidyl epoxies are further classified as glycidyl-ether, glycidyl-ester and glycidyl-amine. The non-glycidyl epoxies are either aliphatic or cycloaliphatic epoxy resins. Bisphenol-A based epoxy resins are the most frequently used epoxy resins available commercially.
The curing process is a chemical reaction in which the epoxy or the ‘oxirane’ ring opens up and reacts with the hardener or curing agent to form three-dimensional, infusible network without forming any byproduct at ambient or elevated temperatures depending upon the nature and use of hardeners. A wide variety of hardeners are available for epoxy resins. The choice of epoxy resin and hardener depends upon the process, desired properties and applications of the cured material. The stoichiometry of the epoxy-hardener system also affects the properties of the cured material. The commonly used curing agents for curing epoxy reins are amines, carboxylic acid anhydrides, polyamides, phenolic resins, isocyanates and polymercaptans. The amine and phenolic resin based hardeners are widely used for curing the epoxy resins.
When anhydrides are used as curing agents or as a hardener, curing accelerators are normally used. The catalyst or accelerator is typically used to accelerate the reaction between epoxy resin and a hardener. Many types of accelerators, such as tertiary amine, boric-acid ester, Lewis acid, organic metal compounds, organic metal salts, and imidazole are widely used.
Resin infusion (RI) or vacuum assisted resin transfer molding (VARTM), is a closed mold process that uses the vacuum bagging technique to compress laminates. Dry laminates are placed in the mold and wet out after the vacuum is applied. After the air is evacuated from the vacuum envelope, the resin is allowed to flow through ports in the vacuum bag into the laminate stacks. This method offers easier handling and fitting of the dry materials in the mold, and the increased compaction produces laminates with low resin content. Epoxy resin-hardener systems for these processes have low viscosity for easy and fast impregnation (less than 300 cP, at processing temperature). Furthermore, these systems also offer several advantages which include fast development of mechanical strength, longer pot life, low exotherm, low vapor pressure, good fiber wetting property, compatibility with consumables and the last but not the least minimum handling risk.